1. Technical Field
This invention relates generally to the field of cable television systems and, more particularly, to apparatus for providing a reverse transmission path from a CATV subscriber to a headend.
2. Description of the Prior Art
At a headend of a cable television system, a scrambler is normally provided to encode premium television channels. The applied scrambling precludes reception by an unauthorized converter/decoder at a connected subscriber premises. Data representing channels or tiers of programming are addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program is authorized and the decoder portion of the converter/decoder will be selectively enabled to decode the scrambled premium channel or program.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/decoder at the premises of the television receiver is particularly expensive. Furthermore, providing the converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive premium channels. As a result, cable television equipment manufacturers have entered into a veritable war with such pirates resulting in complicated service authorization protocols in some instances involving multiple layers of encryption. Decryption and descrambling processes involve transmission of keys for unscrambling and of other forms of service authorization data by both in-band and out-of-band downstream data transmission schemes further increasing the costs of the converter/decoder.
The cable industry has recently resorted to look for new technology and to take a second look at technology developed in the early stages of development of cable television including negative and positive trap technology and interdiction systems to remove equipment off the subscriber's premises. Precipitating the removal of equipment from the subscribers' premises has been the provision of television channel conversion equipment within the new television receivers sold today and the predominance of so-called cable ready receivers. In other words, the converter portions of converter/decoders are in many instances no longer required. Also, removal of equipment from the subscribers' premises accomplishes a removal of equipment from the hands of pirates. Consequently, all forms of equipment known for jamming, scrambling, gating, encoding and encrypting television transmission to the subscriber of a cable television system are being considered for off-premises applications.
Negative traps, for example, are devices located at the drop to a subscriber's dwelling and attenuate a significant portion of a premium television channel rendering that channel unusable by the subscriber. In the conventional embodiment, negative traps are made using L-C filter techniques. The result is a notch with finite quality, Q, and finite shape factor. In the case of a single channel negative trap, the center of the notch is usually located at the picture carrier frequency of the channel to be removed. This technique, sometimes called a static negative trap, requires attenuation at the picture carrier of at least 60 dB to be effective.
Negative traps are usually mounted outside the subscriber's home (typically at the tap) and thereby minimize the exposure associated with placing hardware inside the subscriber's dwelling. Finally, some cable television operators view the negative trap as a more secure means of subscriber control than is sync suppression, as picture reconstruction is viewed as substantially more difficult.
Positive trap systems utilize a narrow band-rejection notch filter just as do negative trap systems. However, unlike negative trap systems which are used to attenuate or trap a premium channel transmission, the notch filter is used to restore the premium television channel. In this scenario, an interfering signal is placed inside the premium television channel at the cable television headend. This interfering signal is then removed at the drop to the subscriber's dwelling by use of the notch filter. Ideally this notch filter removes only the interference without removing a significant amount of television information.
Another form of off-premises equipment is an addressable tap system which was developed by Scientific Atlanta in 1983 or 1984. In this system, an off-premises "tap" located at the drop and addressed by a headend control system, gates a premium channel into the subscriber's premises. The gate may also be addressably closed under headend control to deny service to the subscriber. The addressability of the addressable tap technology has been adapted for switching negative and positive traps in an out of the CATV transmission path. However, all such products did not prove at the time to be viable alternatives to inside-the-home signal descrambler/converters.
A relatively recent technique for premium channel control is the interdiction system, so-called because of the introduction of an interfering signal at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve at least one and up to four or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure. To improve efficiency and to save costs, one oscillator may be used to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The oscillator output jamming signal frequency is periodically moved from channel to channel. Consequently, the oscillator is frequency agile and hops from jamming one premium channel frequency to the next.
Cost reduction is achieved in the systems discussed above, i.e. trap and interdiction systems, by providing shared housings outside the subscribers' premises for each of a plurality of subscriber units, for example, four such units, and common circuitry associated with the several subscriber units as, for example, is provided by FIG. 2 of U.S. Pat. No. 4,912,760. These are mounted out-of-doors on poles or in pedestals for serving especially suburban subscribers. Also, indoor housings are known from so-called multiple dwelling unit systems where a cable television system supplies service to apartment buildings or condominium complexes.
In most if not all of these systems, however, situations arise when it is appropriate to provide a return or reverse path from a particular subscriber to a headend. The need for a reverse path has been recognized since the 1970's. Then, it was envisioned that a number of services could be realized over a cable television distribution plant if a reverse path is provided. The services included remote meter reading, fire and burglar alarm, energy management, home shopping, subscriber polling, and pay-per-view television services. However, in the 1970's and 1980's the preponderance of subscriber service providing equipment was physically located on the subscriber's premises. Consequently, there was no attention paid to a problem today of providing reverse path transmission in an off-premises subscriber equipment environment.
One advance in the art of reverse path transmission from the early days of design of two way addressable cable television systems was the two way distribution amplifier shown in FIG. 4 of the present application. An arrangement is provided for splitting the cable television spectrum into forward and reverse frequency bands. According to a so-called sub-split distribution plant, the frequency band of 54-550 megahertz is reserved for forward, downstream transmission from the headend to the subscriber and the band from 5-30 megahertz is reserved for reverse or upstream transmission. Included in the typical distribution amplifier are diplex filters or diplexers which provide separate paths for the forward and reverse directions on one side and a path for a combined frequency spectrum on the other side.
Typically, a subscriber decoder/converter is provided with a keyboard or other data entry means through which device a subscriber may enter, for example, a home shopping selection or a pay-per-view service request. For remote meter reading, alarm, and energy management services, sensing, measurement, energy control and other devices are appropriately dispersed in a subscriber's premises. Access to the cable distribution plant is provided by a data transmission modem for both subscriber service request data, entered via the keyboard, and utility or alarm data. The data is typically transmitted as frequency shift keyed or phase shift keyed data modulated on a carrier in the 5-30 megahertz band.
As an alternative to FSK or PSK data transmission, spread spectrum techniques and alternative arrangements such as telephone return have been proposed in cable television systems primarily because of the notorious susceptibility of the 5-30 megahertz band to noise and other interferences. A detailed discussion of the problems of return path transmission is provided in U.S. Pat. No. 4,912,721 entitled "Cable Television Spread Spectrum Data Transmission Apparatus."
While arrangements as mentioned above are known for providing on-premises equipment for transmitting data over a reverse path to a headend, there remains a requirement for off-premises reverse path equipment, responsive to the on-premises equipment, for controlling off-premises service providing system equipment such as descrambling or decoding, trap and interdiction system equipment. Furthermore, there remains a requirement in the art of design of cable television equipment for off-premises reverse path equipment responsive to, and which may be shared in common by, a plurality of subscribers and thus provide a cost-effective efficient arrangement. Also, there exists a requirement in the art for off-premises reverse path equipment to provide a bi-directional addressed data transmission path to each subscriber and a uni-directional transmission path toward the headend (relying on existing in-band or out-of-band downstream data transmission).